1. Field of the Invention
The present invention relates to cellular communication systems and, more particularly, to estimating the underlying signal-to-noise ratio and signal-to-interference ratio in a cellular communication system.
2. Description of Related Art
A simplified block diagram of a cellular communication system 10 is shown in FIG. 1. The cellular communication system 10 may be, for example, an IS-54 system. The cellular communication system 10 includes a plurality of base stations 12 used for radio communication with a number of cellular or mobile units 14. The mobile units 14 may be, for example, cellular phones mounted in an automobile, portable hand-held phones, or any other mobile communication devices. Alternatively, the base stations 12 may communicate with a multi-subscriber unit (MSU), such as a residential complex, hotel or office building.
Each base station 12 comprises several transmitters and receivers which simultaneously handle full duplex communications and each is generally located at the center or edge of a geographic coverage region (called a cell). As the mobile units move between cells, the calls are switched or "handed-off" to the appropriate base station.
The base stations 12 are connected (via, for example, telephone lines or microwave links) to a mobile switching center (MSC) 16, which coordinates the routing of all calls in a large geographic service area. The MSC 16 links the base stations 12 and the mobile units 14 to a public switched telephone network (PSTN) 18 and/or another MSC 20.
Communication between a base station 12 and a mobile unit 14 is made via four channels:
(1) Forward Voice Channel (FVC)--voice transmission from base stations to mobile units; PA1 (2) Reverse Voice Channel (RVC)--voice transmission from mobile units to base stations; PA1 (3 & 4) Forward Control Channel (FCC) & Reverse Control Channel (RCC)--for initiating and setting up calls.
Two signals are transmitted on the voice channels along with the voice signals:
(1) Supervisory Audio Tone (SAT): The SAT is one of three tones in the 6 KHz frequency region (5.97 KHz, 6.00 KHz or 6.03 KHz in an AMPS or ETACS system) that is generated and transmitted by a base station on the FVC during a call. The particular frequency of the SAT denotes the particular base station location for a given channel and is assigned by the MSC 16 for each call. The SAT indicates the presence of a mobile call and remains on the entire time the call is in progress.
When a call is set up and a voice channel assignment is issued, the base station 12 immediately begins transmission of the SAT on the FVC by superimposing the SAT on the voice signal. As the mobile unit 14 monitors the FVC, it detects, filters and demodulates the SAT coming from the base station 12 and then reproduces the same tone for continuous transmission back to the base station 12 on the RVC. If the SAT is not present or is improperly detected, both the base station 12 and the mobile unit 14 cease transmission and the MSC 16 uses the vacated channel for new calls.
(2) Signaling Tone (ST): The ST is a 10 KHz data burst which is transmitted by the mobile unit 14 to the base station 12 on the RVC to (a) confirm alert orders sent by the base station 12 that indicate a call to the mobile unit 14, (b) flash for additional services, and (c) notify the base station 12 that the mobile unit 14 has released or terminated the call. The ST is sent simultaneously with the SAT.
The SAT and ST signals will be referred to herein as "tone signals."
In cellular communications systems, it is desirable to know how much noise and/or interference is present in the system. Generally, the noise/interference is measured as a ratio to the signal rate--the signal-to-noise ratio (SNR) and signal-to-interference (SIR) ratio. (The SNR and SIR will be referred to as "noise/interference ratios".) The noise/interference ratios are monitored such that when a ratio is too low for a particular call, the call can be handed off to a different cell. However, if it takes too long to determine the noise/interference ratio, the call may be dropped before the hand-off can occur. Therefore, there remains a need for a system for quickly and accurately estimating the noise/interference ratio in a cellular communication system.